Recently, in devices including semiconductor integrated circuits, in order to improve high integration thereof, the size of each element tends to become miniaturization increasingly. In a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), for example, the thickness of a gate insulating film (gate insulator) becomes less than 10 nm, and therefore it is difficult to ensure resistance to a dielectric breakdown of the insulating film.
The dielectric breakdown of a gate insulating film includes a Time Zero Dielectric Breakdown (TZDB) and a time-dependent dielectric breakdown (TDDB). The TZDB is an initial failure of the gate insulating film, and means a dielectric breakdown in which a large leakage current flows in the insulating film at the moment of applying an electric stress such as a voltage stress, a current stress or the like. On the other hand, the TDDB is a phenomenon in which a dielectric breakdown occurs in the gate insulating film when some time has passed since the application of an electric stress, not at the time point when the electric stress is applied to the gate insulating film.
Further, the TDDB is classified into a hard breakdown (HBD) and a soft breakdown (SBD). The HBD is a well-known dielectric breakdown, and a large leakage current flows in an insulating film after breakdown. On the other hand, the SBD is a state at which a leakage current flows more than at an initial insulating state, but less than at the time after the HBD occurs.
The HBD is a dielectric breakdown that occurs when a relatively high electric stress is applied to an insulating film. Once a leakage current flows when the HBD occurs, an insulating property thereof is never recovered even though the insulating film is left without application of an electric stress thereto thereafter. On the other hand, the SBD is a dielectric breakdown that often occurs when a low electric stress is applied thereto. There is a case in which an insulating property thereof is recovered if the insulating film is left without application of an electric stress thereto after a leakage current occurred. Therefore, a MOSFET in which a SBD occurs may function as a semiconductor device (semiconductor element) although an insulating property thereof becomes unstable. Further, there is a possibility that the SBD shifts to the HBD as time goes by.
In addition, there is a low electric field leakage current referred to as a stress-induced leakage current (SILC) as deterioration after application of the electric stress. The SILC attracts attention as a precursor of the TDDB in addition to its effect on an insulating film to increase a leakage current. In this regard, each of the SILC and SBD still has many unclear points even though various examinations have been carried out. The SBD is also referred to as “B-mode SILC”, and thus the distinction between the SILC and the SBD is unclear.
In these deterioration modes of the insulating film, the SBD and SILC particularly become problems in thinning a gate insulating film. In the case where the thickness of the gate insulating film (gate oxidized film) is 10 nm or less, the deterioration frequently occurs in the low electric field intensity range of 10 MV/cm or less (that is, in the low voltage range in which the electric field intensity is in the range of 10 MV/cm or less), and this becomes major cause that prevent a gate insulating film from being thinned.
For example, Japanese Laid-Open Patent Application No. 2002-299612 discloses an insulating film (gate insulating film) of a semiconductor device in which density of hydrogen atoms is reduced to a predetermined value in order to prevent the occurrence of the SILC. However, the above-mentioned patent application focuses on prevention of occurrence of the SILC, and as a result, the occurrence of the SBD is not examined and discussed in this application. In this regard, each of hydrogen atoms in the insulating film exists at a state of molecular hydrogen or at a connected state to any one of constituent elements of the insulating film. However, this patent application only defines the total amount of hydrogen atoms. According to consideration of the present inventors, it is understood that it is difficult to prevent the occurrence of SBD only by reducing the total amount of hydrogen atoms in the insulating film.